Electrochemical water splitting cell

ABSTRACT

A CoVO x  composite electrode and method of making is described. The composite electrode comprises a substrate with an average 0.5-5 μm thick layer of CoVO x  having pores with average diameters of 2-200 nm. The method of making the composite electrode involves contacting the substrate with an aerosol comprising a solvent, a cobalt complex, and a vanadium complex. The CoVO x  composite electrode is capable of being used in an electrochemical cell for water oxidation.

STATEMENT REGARDING PRIOR DISCLOSURES BY THE INVENTORS

Aspects of this technology are described in an article “DirectDeposition of Amorphous Cobalt-Vanadium Mixed Oxide Films forElectrocatalytic Water Oxidation” by Muhammad Ali Ehsan, Abbas HakeemSaeed, Muhammad Sharif, and Abdul Rehman, in ACS Omega, 2019, 4,12671-12679, DOI: 10.1021/acsomega.9b01385, which is incorporated hereinby reference in its entirety.

STATEMENT OF ACKNOWLEDGEMENT

This project was prepared with support from the Center of Excellence inNanotechnology (CENT) at King Fahd University of Petroleum & Minerals(KFUPM) and from the Deanship of Scientific Research (DSR) at KFUPM:Project no. IN161012.

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to a method of making a CoVO_(x) compositethin film electrode that is capable of electrocatalytic water splitting.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Water splitting reactions for generating and storing clean energy in theform of hydrogen may fulfill rising global energy demands whilemitigating ever-increasing environmental concerns. See Wang, Y.; Suzuki,H.; Xie, J.; Tomita, O.; Martin, D. J.; Higashi, M.; Kong, D.; Abe, R.;Tang, J., Mimicking Natural Photosynthesis: Solar to Renewable H2 FuelSynthesis by Z-Scheme Water Splitting Systems. Chemical Reviews 2018,118 (10), 5201-5241; and Chu, S.; Cui, Y.; Liu, N., The path towardssustainable energy. Nature Materials 2016, 16, 16, each incorporatedherein by reference in their entirety. A bottle neck here is the fourelectron oxygen evolution reaction (OER), which due to its sluggishkinetics and high overvoltage, requires highly active catalyticmaterials for an economically viable rate of reaction. See Reier, T.;Oezaslan, M.; Strasser, P., Electrocatalytic Oxygen Evolution Reaction(OER) on Ru, Ir, and Pt Catalysts: A Comparative Study of Nanoparticlesand Bulk Materials. ACS Catalysis 2012, 2 (8), 1765-1772; Reier, T.;Nong Hong, N.; Teschner, D.; Schlögl, R.; Strasser, P., ElectrocatalyticOxygen Evolution Reaction in Acidic Environments—Reaction Mechanisms andCatalysts. Advanced Energy Materials 2016, 7 (1), 1601275; and Kumar,A.; Ciucci, F.; Morozovska, A. N.; Kalinin, S. V.; Jesse, S., Measuringoxygen reduction/evolution reactions on the nanoscale. Nature Chemistry2011, 3, 707, each incorporated herein by reference in their entirety.At the same time, these materials need to be robust, efficient, andfacile to be produced. Current benchmarks for water oxidation are IrO₂and RuO₂, but these are based on scarce and costly noble metals, andthus replacing them with earth abundant materials is an active area ofresearch. See McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T.F., Benchmarking Heterogeneous Electrocatalysts for the Oxygen EvolutionReaction. Journal of the American Chemical Society 2013, 135 (45),16977-16987; Lyons, M. E. G.; Brandon, M. P., A comparative study of theoxygen evolution reaction on oxidised nickel, cobalt and iron electrodesin base. Journal of Electroanalytical Chemistry 2010, 641 (1), 119-130;Faber, M. S.; Jin, S., Earth-abundant inorganic electrocatalysts andtheir nanostructures for energy conversion applications. Energy &Environmental Science 2014, 7 (11), 3519-3542; and Roger, I.; Shipman,M. A.; Symes, M. D., Earth-abundant catalysts for electrochemical andphotoelectrochemical water splitting. Nature Reviews Chemistry 2017, 1,0003, each incorporated herein by reference in their entirety.

While the approach to develop these electrocatalysts mostly remainsempirical, some attempts to explore theoretical guidelines have alsobeen made. See Grimaud, A.; May, K. J.; Carlton, C. E.; Lee, Y. L.;Risch, M.; Hong, W. T.; Zhou, J.; Shao-Hom, Y., Double perovskites as afamily of highly active catalysts for oxygen evolution in alkalinesolution. Nat Commun 2013, 4, 2439; and Suntivich, J.; May, K. J.;Gasteiger, H. A.; Goodenough, J. B.; Shao-Hom, Y., A perovskite oxideoptimized for oxygen evolution catalysis from molecular orbitalprinciples. Science 2011, 334 (6061), 1383-5, incorporated herein byreference in their entirety.

In one such purely descriptor approach, the intrinsic activity of themixed metal oxide films is correlated to the M-OH bond strengths usingvolcano plots. See Morales-Guio, C. G.; Thorwarth, K.; Niesen, B.;Liardet, L.; Patscheider, J.; Ballif, C.; Hu, X., Solar HydrogenProduction by Amorphous Silicon Photocathodes Coated with a MagnetronSputter Deposited Mo₂C Catalyst. J Am Chem Soc 2015, 137 (22), 7035-8,incorporated herein by reference in its entirety. The outcome of thisapproach is quite debatable as it neither describes the physical originnor the nature of the active sites in the metal oxide films. SeeStevens, M. B.; Trang, C. D. M.; Enman, L. J.; Deng, J.; Boettcher, S.W., Reactive Fe-Sites in Ni/Fe (Oxy)hydroxide Are Responsible forExceptional Oxygen Electrocatalysis Activity. J Am Chem Soc 2017, 139(33), 11361-11364; and Friebel, D.; Louie, M. W.; Bajdich, M.; Sanwald,K. E.; Cai, Y.; Wise, A. M.; Cheng, M.-J.; Sokaras, D.; Weng, T.-C.;Alonso-Mori, R.; Davis, R. C.; Bargar, J. R.; Norskov, J. K.; Nilsson,A.; Bell, A. T., Identification of Highly Active Fe Sites in (Ni,Fe)OOHfor Electrocatalytic Water Splitting. Journal of the American ChemicalSociety 2015, 137 (3), 1305-1313, each incorporated herein by referencein their entirety. However, it approximates the superior catalyticactivity of films like NiFeO_(x) and CoFeO_(x), explaining that Ni andCo are located on different branches of the volcano plot as compared toFe, thereby benefiting from the balancing of M-OH bond strengths forhigher catalytic activity. Even better performance has been shown byCoVO_(x) catalysts, especially ones having amorphous character, with Vsitting in the same branch of volcano plot as Fe while Co sits atexactly opposite branch, as depicted in FIG. 1 . See Liu, J.; Ji, Y.;Nai, J.; Niu, X.; Luo, Y.; Guo, L.; Yang, S., Ultrathin amorphouscobalt-vanadium hydr(oxy)oxide catalysts for the oxygen evolutionreaction. Energy & Environmental Science 2018, 11 (7), 1736-1741;Thorat, G. M.; Jadhav, H. S.; Roy, A.; Chung, W.-J.; Seo, J. G., DualRole of Deep Eutectic Solvent as a Solvent and Template for theSynthesis of Octahedral Cobalt Vanadate for an Oxygen EvolutionReaction. ACS Sustainable Chemistry & Engineering 2018, 6 (12),16255-16266; and Xing, Z.; Wu, H.; Wu, L.; Wang, X.; Zhong, H.; Li, F.;Shi, J.; Song, D.; Xiao, W.; Jiang, C.; Ren, F., A multifunctionalvanadium-doped cobalt oxide layer on silicon photoanodes for efficientand stable photoelectrochemical water oxidation. Journal of MaterialsChemistry A 2018, 6 (42), 21167-21177, each incorporated herein byreference in their entirety.

Thus, a recent trend is the straightforward, low temperature, and fastfabrication of Co—V mixed oxide films having amorphous character toprovide abundant defects in a distinctive molecular structure. SeeChakrapani, K.; Bendt, G.; Hajiyani, H.; Lunkenbein, T.; Greiner, M. T.;Masliuk, L.; Salamon, S.; Landers, J.; Schlogl, R.; Wende, H.;Pentcheva, R.; Schulz, S.; Behrens, M., The Role of Composition ofUniform and Highly Dispersed Cobalt Vanadium Iron Spinel Nanocrystalsfor Oxygen Electrocatalysis. ACS Catalysis 2018, 8 (2), 1259-1267; andPeng, X.; Wang, L.; Hu, L.; Li, Y.; Gao, B.; Song, H.; Huang, C.; Zhang,X.; Fu, J.; Huo, K.; Chu, P. K., In situ segregation of cobaltnanoparticles on VN nanosheets via nitriding of CO₂V₂O₇ nanosheets asefficient oxygen evolution reaction electrocatalysts. Nano Energy 2017,34, 1-7, each incorporated herein by reference in their entirety.Accordingly, Liardet et al synthesized amorphous electrocatalysts basedon Co—V mixed oxides with different metallic ratios while approximatingtheir position on the volcano plots, and showed the highly active natureof the resulting materials (e.g., a-Co_(0.58)V_(0.42)O_(x)) whendeposited over glassy carbon electrodes and nickel foams. See Liardet,L.; Hu, X., Amorphous Cobalt Vanadium Oxide as a Highly ActiveElectrocatalyst for Oxygen Evolution. ACS Catalysis 2018, 8 (1),644-650, incorporated herein by reference in its entirety. Liu et al hasalso shown a similar catalytic activity of amorphous Co—V (hydroxy)oxideultrathin films when supported on gold foams.

Typically, two strategies are implemented in the rational design ofamorphous catalytic materials: (i) a solid-state reaction (SSR) routeusing pure metals or metal salts, and (ii) wet chemistry syntheticmethods such as hydrothermal synthesis or co-precipitation techniques.See Schmalzried, H., Solid-State Reactions. Angewandte ChemieInternational Edition in English 1963, 2 (5), 251-254; Jiang, X.; Zhang,T.; Lee, J. Y., A Polymer-Infused Solid-State Synthesis of a LongCycle-Life Na₃V₂(PO₄)₃/C Composite. ACS Sustainable Chemistry &Engineering 2017, 5 (9), 8447-8455; Liardet et al. (2018); Kim, J. S.;Kim, S. Y.; Kim, D. H.; Ott, R. T.; Kim, H. G.; Lee, M. H., Effect ofhydrothermal condition on the formation of multi-component oxides ofNi-based metallic glass under high temperature water near the criticalpoint. AIP Advances 2015, 5 (7), 077132; Liu et al. (2018); and Dolla,T. H.; Billing, D. G.; Sheppard, C.; Prinsloo, A.; Carleschi, E.; Doyle,B. P.; Pruessner, K.; Ndungu, P., Mn substituted Mn_(x)Zn_(1-x)Co₂O₄oxides synthesized by co-precipitation; effect of doping on thestructural, electronic and magnetic properties. RSC Advances 2018, 8(70), 39837-39848, each incorporated herein by reference in theirentirety. The SSR routes are limited by a high temperature processingand long reaction times because of the long diffusion distances. SeeFister, L.; Johnson, D. C., Controlling solid-state reaction mechanismsusing diffusion length in ultrathin-film superlattice composites.Journal of the American Chemical Society 1992, 114 (12), 4639-4644,incorporated herein by reference in its entirety. Still, the procedurehas a lesser control on the size and morphology of the final product.Typically, SSR for cobalt vanadate is performed at high temperaturesof >720° C. for as long as 40 h using vanadium oxides and hydratedcobalt oxalates. On the other hand, wet chemical methods providecontrollable synthesis and intriguing morphologies with productformation occurring at relatively low temperatures, yet require longreaction times and expensive instruments such as high pressure reactors.Further adding to this laborious work is the coating of resultingproducts onto substrates in separate manipulation steps. Therefore, thescale up of the final product becomes a limiting factor for theseprocesses.

In view of the forgoing, one objective of the present invention is toprovide an aerosol assisted chemical vapor deposition (AACVD) protocol,which uses solution-based precursors with a deposition step on apre-heated substrate. During deposition via AACVD, the particle growthand sintering processes simultaneously occur on the substrate surface todevelop well interconnected morphological features and produce adhesivefilm electrodes in a matter of minutes. This was used to generate filmsof Co—V mixed oxide for effective and stable water oxidation.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect, the present disclosure relates to acomposite thin film electrode, which comprises a CoVO_(x) layer havingan average thickness of 500 nm-5 μm in contact with a substrate, theCoVO_(x) layer comprising amorphous CoVO_(x) having a Co:V molar ratioin a range of 1.0:1.2-1.5:1.0.

In one embodiment, the CoVO_(x) layer is porous with a pore size in arange of 2-200 nm.

In one embodiment, the composite thin film electrode has anelectrochemically active surface area in a range of 12-22 mF/cm².

In one embodiment, the CoVO_(x) layer consists essentially of amorphousCoVO_(x).

In one embodiment, the CoVO_(x) layer has an O:Co molar ratio in a rangeof 4:1 to 9:1.

In one embodiment, the substrate is a transparent conducting filmselected from the group consisting of FTO, ITO, AZO, GZO, IZO, IZTO,IAZO, IGZO, IGTO, and ATO.

According to a second aspect, the present disclosure relates to a methodof making the composite thin film electrode of the first aspect. Themethod involves contacting an aerosol with the substrate to deposit theCoVO_(x) layer to form the composite thin film electrode. The aerosolcomprises a carrier gas, and a cobalt complex and a vanadium complexdissolved in a solvent. The substrate has a temperature in a range of425-525° C. during the contacting.

In one embodiment, the cobalt complex and the vanadium complex eachindependently comprise at least one ligand selected from the groupconsisting of acetylacetonate, acetate ligand, trifluoroacetate,isopropanol, and tetrahydrofuran.

In one embodiment, the cobalt complex is Co(II) acetylacetonate, and thevanadium complex is V(III) acetylacetonate.

In one embodiment, before the contacting, the aerosol consistsessentially of the carrier gas, the cobalt complex, the vanadiumcomplex, and the solvent.

In one embodiment, a weight ratio of the cobalt complex to the solventin the aerosol, and/or a weight ratio of the vanadium complex to thesolvent in the aerosol is in a range of 1:1,000-1:5.

In one embodiment, the aerosol is contacted with the substrate for atime period of 10-30 min.

In one embodiment, the carrier gas has a flow rate in a range of 20-250cm³/min during the contacting.

According to a third aspect, the present disclosure relates to anelectrochemical cell comprising the composite thin film electrode of thefirst aspect, a counter electrode, and

an electrolyte solution in contact with both electrodes.

In one embodiment, the composite thin film electrode has anoverpotential in a range of 270-335 mV at a current density of 9-11mA/cm².

In one embodiment, the composite thin film electrode has a currentdensity of 1.0-10.0 mA/cm² when the electrodes are subjected to a biaspotential of 1.45-1.55 V.

In one embodiment, the electrolyte solution comprises water and aninorganic base having a concentration of 0.1-1.0 M.

In one embodiment, the composite thin film electrode has a mass activityin range of 38-50 A/g at 350 mV.

According to a fourth aspect, the present disclosure relates to a methodfor decomposing water into H₂ and O₂. This involves subjecting theelectrodes of the electrochemical cell of claim 14 with a potential of0.5-2.0 V.

In one embodiment, the method also involves separately collectingH₂-enriched gas and O₂-enriched gas.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a volcano plot describing mass activity of oxygen evolutionreactions against M-OH bond strength.

FIG. 2A is a large area, low resolution (10K×) FESEM of the CoVO_(x)-20film.

FIG. 2B is a high resolution (50K×) FESEM of the CoVO_(x)-20 film.

FIG. 2C is a cross-section FESEM of the CoVO_(x)-20 film on a FTO glasssubstrate.

FIG. 2D is a large area, low resolution (10K×) FESEM of the CoVO_(x)-40film.

FIG. 2E is a high resolution (50K×) FESEM of the CoVO_(x)-40 film.

FIG. 2F is a cross-section FESEM of the CoVO_(x)-40 film on a FTO glasssubstrate.

FIG. 2G is a large area, low resolution (10K×) FESEM of the CoVO_(x)-60film.

FIG. 2H is a high resolution (50K×) FESEM of the CoVO_(x)-60 film.

FIG. 2I is a cross-section FESEM of the CoVO_(x)-60 film on a FTO glasssubstrate.

FIG. 3A is EDX spectra of the CoVO_(x)-20 film.

FIG. 3B is the atomicity recorded of the CoVO_(x)-20 film of FIG. 3A.

FIG. 3C is an FESEM image of the analysis area of the CoVO_(x)-20 filmused for FIG. 3A.

FIG. 3D is EDX spectra of the CoVO_(x)-40 film.

FIG. 3E is the atomicity recorded of the CoVO_(x)-40 film of FIG. 3D.

FIG. 3F is an FESEM image of the analysis area of the CoVO_(x)-40 filmused for FIG. 3A.

FIG. 3G is EDX spectra of the CoVO_(x)-60 film.

FIG. 3H is the atomicity recorded of the CoVO_(x)-60 film of FIG. 3G.

FIG. 3I is an FESEM image of the analysis area of the CoVO_(x)-60 filmused for FIG. 3G.

FIG. 4A is a high resolution XPS of the CoVO_(x)-20 film showing thebinding energies for Co 2p.

FIG. 4B is a high resolution XPS of the CoVO_(x)-20 film showing thebinding energies for V 2p.

FIG. 4C is a high resolution XPS of the CoVO_(x)-20 film showing thebinding energies for O 1s.

FIG. 5A is a graph of the forward potential sweeps for fabricated mixedoxide electrocatalytic materials in 0.5 M KOH electrolyte solution andat the scan rate of 10 mV s⁻¹.

FIG. 5B is a graph of the overpotential values of the same films of FIG.5A at a current density of 10 mA cm².

FIG. 6A shows LSV curves for the CoVO_(x)-20 film at different scanrates.

FIG. 6B shows a zoomed in LSV curve for the water oxidation reaction ata scan rate of 1 mV sec⁻¹.

FIG. 7 is a graph illustrating the Tafel plots for different films at ascan rate of 10 mV/sec as well as CoVO_(x)-20 film at a scan rate of 1.0mV/sec in 0.5 M KOH electrolyte solution.

FIG. 8A shows the long term stability tests of the prepared films atconstant current density of 20 mA/cm² for more than 5 h and a constantcurrent density of 100 mA/cm² for more than 5 h.

FIG. 8B shows the first and the 500th scan of LSV measurement with nosignificant changes in overvoltage and current density.

FIG. 9 is a plausible reaction mechanism for electroxidation of water inthe absence and presence of vanadium in a cobalt electrocatalyst.

FIG. 10 is a schematic of the AACVD setup used for the synthesis of theCoVO_(x) films.

FIG. 11 shows XRD patterns of Co—V mixed oxide film electrodes,CoVO_(x)-20, CoVO_(x)-40, and CoVO_(x)-60 fabricated in 20, 40, and 60min of deposition time at 475° C.

FIG. 12A shows XRD patterns from films produced using the Co(acac)₂precursor and without the V(acac)₃ precursor.

FIG. 12B shows XRD patterns from films produced using the V(acac)₃precursor and without the Co(acac)₂ precursor.

FIG. 13A shows an FESEM image of the CoVO_(x)-20 film.

FIG. 13B is an EDX mapping of VKa1 obtained from the region shown inFIG. 13A.

FIG. 13C is an EDX mapping of CoKa1 obtained from the region shown inFIG. 13A.

FIG. 13D shows an FESEM image of the CoVO_(x)-40 film.

FIG. 13E is an EDX mapping of VKa1 obtained from the region shown inFIG. 13D.

FIG. 13F is an EDX mapping of CoKa1 obtained from the region shown inFIG. 13D.

FIG. 13G shows an FESEM image of the CoVO_(x)-60 film.

FIG. 13H is an EDX mapping of VKa1 obtained from the region shown inFIG. 13G.

FIG. 13I is an EDX mapping of CoKa1 obtained from the region shown inFIG. 13G.

FIG. 14 shows the current density vs. the scan rate for experiments inthe non-faradaic zone, and the calculated slope values for the threedifferent films.

FIG. 15 shows a Nyquist plot for CoVO_(x)-20, CoVO_(x)-40, andCoVO_(x)-60 films at an applied potential of 1.48 V vs. RHE in thefrequency range of 0.1 Hz to 100 KHz.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present disclosure will now be described more fullyhereinafter with reference to the accompanying drawings, in which some,but not all embodiments of the disclosure are shown.

The present disclosure will be better understood with reference to thefollowing definitions. As used herein, the words “a” and “an” and thelike carry the meaning of “one or more.” Within the description of thisdisclosure, where a numerical limit or range is stated, the endpointsare included unless stated otherwise. It will be further understood thatthe terms “comprises” and/or “comprising,” when used in thisspecification, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

As used herein, the words “about,” “approximately,” or “substantiallysimilar” may be used when describing magnitude and/or position toindicate that the value and/or position described is within a reasonableexpected range of values and/or positions. For example, a numeric valuemay have a value that is +/−0.1% of the stated value (or range ofvalues), +/−1% of the stated value (or range of values), +/−2% of thestated value (or range of values), +/−5% of the stated value (or rangeof values), +/−10% of the stated value (or range of values), +/−15% ofthe stated value (or range of values), or +/−20% of the stated value (orrange of values). Within the description of this disclosure, where anumerical limit or range is stated, the endpoints are included unlessstated otherwise. Also, all values and subranges within a numericallimit or range are specifically included as if explicitly written out.

As used herein, “compound” is intended to refer to a chemical entity,whether as a solid, liquid, or gas, and whether in a crude mixture orisolated and purified.

As used herein, “composite” refers to a combination of two or moredistinct constituent materials into one. The individual components, onan atomic level, remain separate and distinct within the finishedstructure. The materials may have different physical or chemicalproperties, that when combined, produce a material with characteristicsdifferent from the original components. In some embodiments, a compositemay have at least two constituent materials that comprise the sameempirical formula but are distinguished by different densities, crystalphases, or a lack of a crystal phase (i.e. an amorphous phase).

The present disclosure is intended to include all hydration states of agiven compound or formula, unless otherwise noted or when heating amaterial. For example, Ni(NO₃)₂ includes anhydrous Ni(NO₃)₂,Ni(NO₃)₂·6H₂O, and any other hydrated forms or mixtures. CuCl₂ includesboth anhydrous CuCl₂ and CuCl₂·2H₂O.

In addition, the present disclosure is intended to include all isotopesof atoms occurring in the present compounds and complexes. Isotopesinclude those atoms having the same atomic number but different massnumbers. By way of general example, and without limitation, isotopes ofhydrogen include deuterium and tritium. Isotopes of carbon include ¹³Cand ¹⁴C. Isotopes of nitrogen include ¹⁴N and ¹⁵N. Isotopes of oxygeninclude ¹⁶O, ¹⁷O, and ¹⁸O. Isotopes of cobalt include ⁵⁶Co, ⁵⁷Co, ⁵⁸Co,⁵⁹Co, and ⁶⁰Co. Isotopes of vanadium include ⁴⁸V, ⁹V, ⁵⁰V, and ⁵¹V.Isotopically-labeled compounds of the disclosure may generally beprepared by conventional techniques known to those skilled in the art orby processes analogous to those described herein, using an appropriateisotopically-labeled reagent in place of the non-labeled reagentotherwise employed.

As defined here, an aerosol is a suspension of solid or liquid particlesin a gas. An aerosol includes both the particles and the suspending gas.Primary aerosols contain particles introduced directly into the gas,while secondary aerosols form through gas-to-particle conversion. Thereare several measures of aerosol concentration. Environmental science andhealth fields often use the mass concentration (M), defined as the massof particulate matter per unit volume with units such as μg/m³. Alsocommonly used is the number concentration (N), the number of particlesper unit volume with units such as number/m³ or number/cm³. The size ofparticles has a major influence on their properties, and the aerosolparticle radius or diameter (d_(P)) is a key property used tocharacterize aerosols. Aerosols vary in their dispersity. A monodisperseaerosol, producible in the laboratory, contains particles of uniformsize. Most aerosols, however, as polydisperse colloidal systems, exhibita range of particle sizes. Liquid droplets are almost always nearlyspherical, but scientists use an equivalent diameter to characterize theproperties of various shapes of solid particles, some very irregular.The equivalent diameter is the diameter of a spherical particle with thesame value of some physical property as the irregular particle. Theequivalent volume diameter (d_(e)) is defined as the diameter of asphere of the same volume as that of the irregular particle. Alsocommonly used is the aerodynamic diameter. The aerodynamic diameter ofan irregular particle is defined as the diameter of the sphericalparticle with a density of 1000 kg/m³ and the same settling velocity asthe irregular particle.

As defined here, an electrode is an electrically conductive materialcomprising a metal and is used to establish electrical contact with anonmetallic part of a circuit. An “electrically-conductive material” asdefined here is a substance with an electrical resistivity of at most10⁻⁶ Ω·m, preferably at most 10⁻⁷ Ω·m, more preferably at most 10⁻⁸ Ω·mat a temperature of 20-25° C. The electrically-conductive materialcomprise platinum-iridium alloy, iridium, titanium, titanium alloy,stainless steel, gold, cobalt alloy, copper, aluminum, tin, iron, and/orsome other metal.

According to a first aspect, the present disclosure relates to acomposite thin film electrode. The composite thin film electrodecomprises a CoVO_(x) layer having an average thickness in a range of 500nm-5 μm, preferably 700 nm-4 μm, more preferably 800 nm-3 μm, even morepreferably 900 nm-2 μm, or about 1 μm. In one embodiment, the thicknessof the layer may vary from location to location on the electrode by1-30%, preferably 5-20%, relative to the average thickness of the layer.The CoVO_(x) layer is a mixed metal oxide and may also be called acobalt vanadium oxide layer, or a vanadium cobalt oxide layer.

In one embodiment, the CoVO_(x) layer comprises amorphous CoVO_(x). Theamount of amorphous CoVO_(x) may be measured by X-ray diffractionpatterns. In one embodiment, the CoVO_(x) layer consists essentially ofamorphous CoVO_(x), meaning that the CoVO_(x) layer comprises at least99 wt %, preferably 99.9 wt %, more preferably 99.95 wt % CoVO_(x) in anamorphous (non-crystalline) state, relative to a total weight of theCoVO_(x) layer.

While the name “CoVO_(x)” when read literally implies a 1:1 molar ratioof Co:V, in some embodiments, this molar ratio may not be 1:1. In oneembodiment, the CoVO_(x) layer comprises or consists essentially ofCoVO_(x) having a Co:V molar ratio in a range of 1.0:1.2-1.5:1.0,preferably 1.0:1.2-1.2:1.0 or 1.0:1.2-1.1:1.0, more preferably1.0:1.18-1.05:1.0, or about 1.0:1.16, or about 1:1. Here, the CoVO_(x)layer consisting essentially of CoVO_(x) means that at least 99 wt %,preferably at least 99.9 wt %, more preferably at least 99.95 wt %, orabout 100 wt % of the CoVO_(x) layer, relative to a total weight iseither cobalt, vanadium, or oxygen.

In one embodiment, the CoVO_(x) layer has an O:Co molar ratio in a rangeof 4:1 to 9:1, preferably 5:1-8.5:1, more preferably 6:1-8:1, or about7.8:1. In one embodiment, the CoVO_(x) layer has an O:V molar ratio in arange of 4:1 to 9:1, preferably 5:1-8:1, more preferably 6:1-7:1, orabout 6.7:1.

The CoVO_(x) layer may be in the form of a mesh, exfoliated surface,and/or blistered surface. FIG. 2B shows one such embodiment. TheCoVO_(x) layer may have pores or open spaces. In one embodiment, theCoVO_(x) layer may have a porosity in a range of 10-70%, preferably20-60%. In a related embodiment, the CoVO_(x) layer may have a surfacearea per mass CoVO_(x) layer of 80-350 m²/g, preferably 100-250 m²/g,even more preferably 120-220 m²/g. In an alternative embodiment, theCoVO_(x) layer may be in the form of particles, cylinders, boxes,spikes, flakes, plates, ellipsoids, toroids, stars, ribbons, discs,rods, granules, prisms, cones, flakes, platelets, sheets, or some othershape.

In one embodiment, the CoVO_(x) layer is porous with a pore size in arange of 2-200 nm, preferably 2-100 nm, more preferably 3-50 nm, evenmore preferably 3-10 nm, or 2-10 nm, or 2-5 nm. In one embodiment, theCoVO_(x) layer is monolithic, meaning that all parts of the layer areattached to one another as a single structure, as opposed to the layerbeing fragmented or in the form of particles. In one embodiment, theCoVO_(x) layer is in the form of a mesh having strands or wires ofCoVO_(x) with diameters in a range of 100-250 nm, 120-220 nm, thestrands or wires being joined together. FIG. 2B shows one such example.

In one embodiment, the pores of the CoVO_(x) layer are monodisperse indiameter, having a coefficient of variation or relative standarddeviation, expressed as a percentage and defined as the ratio of thepore diameter standard deviation (a) to the pore diameter mean (p),multiplied by 100%, of less than 25%, preferably less than 10%,preferably less than 8%, preferably less than 6%, preferably less than5%. In a preferred embodiment, the pores are monodisperse having a porediameter distribution ranging from 80% of the average pore diameter to120% of the average pore diameter, preferably 85-115%, preferably90-110% of the average pore diameter. In another embodiment, the porediameters are not monodisperse.

In one embodiment, the CoVO_(x) layer is in contact with a substrate. Inone embodiment, the substrate is a transparent conducting film selectedfrom the group consisting of FTO (fluorine-doped tin oxide), ITO (indiumtin oxide), AZO (aluminum-doped zinc oxide), GZO (gallium-doped zincoxide), IZO (indium zinc oxide), IZTO (indium zinc tin oxide), IAZO(indium aluminum zinc oxide), IGZO (indium gallium zinc oxide), IGTO(indium gallium tin oxide), and ATO (antimony tin oxide). In otherembodiments, transparent conducting polymers (such as PEDOT) or carbonnanotubes may be used with or in place of the compounds previouslymentioned. In a preferred embodiment, the substrate is FTO. Thetransparent conducting film may have an average thickness of 1 μm-1 mm,preferably 10 μm-900 μm, more preferably 200 μm-800 μm, or about 600 μm.Alternatively, the transparent conducting film may have an averagethickness of 500 nm-200 μm, preferably 1 μm-100 μm, more preferably 10μm-50 μm. However, in some embodiments, the transparent conducting filmmay have an average thickness of less than 500 nm. For instance, thetransparent conducting film may have an average thickness of 50-500 nm,80-300 nm, or 100-250 nm. Preferably the transparent conducting film isattached to an additional support, such as a glass slide. However, inother embodiments, the substrate may be glass, quartz, ceramic, a metal,a composite material, or a polymeric material having temperatureresistance at least up to the temperature of the substrate heating.Where the substrate comprises glass, the glass may beboro-aluminosilicate glass, sodium borosilicate glass, fused-silicaglass, soda lime glass, or some other type of glass.

In a preferred embodiment, the substrate is substantially flat, smooth,and planar. Here, the substrate may have a thickness at any and everypoint on the substrate that varies by less than 4 nm, preferably by lessthan 3 nm, more preferably by less than 2 nm, even more preferably byless than 1 nm, less than 0.5 nm, or less than 0.3 nm than the averagethickness. In one embodiment, the substrate is smooth and without pores,nanostructures, or microstructures. In a related embodiment, thesubstrate is rectangular and essentially smooth so that the substrateand a three dimensional convex hull of the substrate occupy essentiallythe same volume. In another related embodiment, the bulk volume of thesubstrate is the same as its actual volume, its actual volume beingmeasured by fluid displacement or some other method.

In one embodiment, the substrate has a sheet resistance in a range of1-40 Ω sq⁻¹, preferably 2-20 Ω sq⁻¹, more preferably 4-12 Ω sq⁻¹, orabout 8 Ω sq⁻¹. Preferably, the CoVO_(x) layer in contact with thesubstrate forms an electrically-conductive material with the transparentconducting film. An “electrically-conductive material” as defined hereis a substance with an electrical resistivity of at most 10⁻⁶ Ω·m,preferably at most 10⁻⁷ Ω·m, more preferably at most 10⁻⁸ Ω·m at atemperature of 20-25° C.

In one embodiment, the composite thin film electrode has anelectrochemically active surface area (ECSA, or electroactive surfacearea) in a range of 12-22 mF/cm², preferably 13-20 mF/cm², morepreferably 14-19 mF/cm², or about 17.6 mF/cm².

In one embodiment, the composite thin film electrode does not comprise anickel foam, a gold foam, or some other metallic foam. In anotherembodiment, the composite thin film electrode does not comprise carbon.In one embodiment, the composite thin film electrode consistsessentially of Co, V, O, and FTO coated glass.

According to a second aspect, the present disclosure relates to a methodof making the composite thin film electrode of the first aspect. Thismethod involves contacting an aerosol with a substrate to deposit theCoVO_(x) layer to form the composite thin film electrode. As describedhere, “contacting an aerosol with a substrate” is considered to besynonymous with “contacting a substrate with an aerosol.” Both phrasesmean that the substrate is exposed to the aerosol, so that a portion ofthe suspended particles of the aerosol directly contact the substrate.Contacting may also be considered equivalent to “introducing” or“depositing,” such as “depositing an aerosol on a substrate.” In oneembodiment, the contacting may be considered aerosol-assisted chemicalvapor deposition (AACVD). In one embodiment, the method of making thecomposite thin film electrode may be considered a one-step method, asthe formation of the CoVO_(x) layer takes place immediately followingand/or during the contacting of the aerosol with the substrate.

In one embodiment, the temperature of the substrate during thecontacting is in a range of 425-550° C., preferably 450-525° C., morepreferably 460-500° C., even more preferably 465-480° C., or about 475°C. In one embodiment, the temperature of the substrate during thecontacting never reaches a temperature of greater than 550° C.,preferably no greater than 500° C., more preferably no greater than 480°C.

The aerosol comprises a carrier gas, a cobalt complex, a vanadiumcomplex, and a solvent. In one embodiment, the aerosol consists of, orconsists essentially of, a carrier gas, a cobalt complex, a vanadiumcomplex, and a solvent before the contacting, preferably immediatelybefore the contacting. Preferably, the cobalt complex and vanadiumcomplex are dissolved or dispersed in the solvent. In some embodiments,the cobalt complex and vanadium complex are dissolved in the sameaerosol droplets. In other embodiments, some aerosol droplets mayconsist of the cobalt complex and solvent, and other aerosol dropletsmay consist of vanadium complex and solvent. Similarly, some aerosoldroplets may consist of only solvent.

In one embodiment, the cobalt complex has an acetylacetone oracetylacetonate (acac) ligand, a trifluoroacetate (TFA) ligand, anacetate ligand (OAc), an isopropanol (^(i)PrOH) ligand, atetrahydrofuran (THF) ligand, and/or a water (H₂O) ligand. In oneembodiment, a molar ratio of acetylacetonate ligands to Co in the cobaltcomplex is in a range of 1:1-3:1, or about 2:1. In one embodiment, thecobalt complex is Co(II) acetylacetonate, or Co(acac)₂. In alternativeembodiments, the cobalt complex may be bromopentaamminecobalt(III)bromide, caesium hexafluorocobaltate(IV), chloro(pyridine)cobaloxime,chloropentamminecobalt chloride,cis-dichlorobis(ethylenediamine)cobalt(III) chloride,trans-dichlorobis(ethylenediamine)cobalt(III) chloride,hexamminecobalt(III) chloride, nitropentaamminecobalt(III) chloride,tetracobalt dodecacarbonyl, tris(ethylenediamine)cobalt(III) chloride orsome other cobalt complex or cobalt salt. In these alternativeembodiments, the cobalt may have a II, III, or IV oxidation state.

In one embodiment, the vanadium complex has an acetylacetone oracetylacetonate (acac) ligand, a trifluoroacetate (TFA) ligand, anacetate ligand (OAc), an isopropanol (^(i)PrOH) ligand, atetrahydrofuran (THF) ligand, and/or a water (H₂O) ligand. In oneembodiment, the vanadium complex is V(III) acetylacetonate, or V(acac)₂.In one embodiment, a molar ratio of acetylacetone ligands to V in thevanadium complex is in a range of 1:1-4:1, 2:1-4:1, or about 3:1. Inalternative embodiments, without limitation, the vanadium complex may bevanadium acetylacetonate, vanadium hexacarbonyl, vanadocene, vanadylperchlorate, vanadyl acetylacetonate, ammonium metavanadate, vanadocenedichloride, or some other vanadium complex or vanadium salt, such as avanadium halide. In these alternative embodiments, the vanadium may havea II, III, IV, or V oxidation state.

In alternative embodiments, the cobalt complex and/or the vanadiumcomplex may not comprise one or more of the ligands acetylacetonate,trifluoroacetate, acetate, isopropanol, tetrahydrofuran, or water, andin other embodiments, one or more ligands may be substituted with otherligands, such as ethanol. In one embodiment, other ligands may bepresent in the cobalt complex and/or the vanadium complex, including butnot limited to acetonitrile, methyl isocyanide, phosphine, bipyridine,nitrilotriacetic acid, and diimine.

In one embodiment, the cobalt complex and the solvent are present in theaerosol at a cobalt complex to solvent weight ratio of 1:1,000-1:5,preferably 1:800-1:50, more preferably 1:600-1:70, even more preferably1:500-1:100, or about 1:365.

In one embodiment, the vanadium complex and the solvent are present inthe aerosol at a vanadium complex to solvent weight ratio of1:1,000-1:5, preferably 1:800-1:50, more preferably 1:600-1:70, evenmore preferably 1:500-1:100, or about 1:322.

In one embodiment, the cobalt complex and the vanadium complex arepresent in the aerosol at a cobalt to vanadium molar ratio of1.0:1.2-1.5:1.0, preferably 1.0:1.2-1.4:1.0, or 1.0:1.2-1.2:1.0, or1.0:1.2-1.1:1.0, more preferably 1.0:1.18-1.05:1.0, or about 1.0:1.16,or about 1.36:1.0, or about 1.16:1.0, or about 1:1.

In an alternative embodiment, rather than a cobalt complex and avanadium complex existing as separate molecules, a single moleculecomprising both vanadium and cobalt may be used in the aerosol.

In one embodiment, the carrier gas is N₂, He, compressed air, and/or Ar.Preferably the carrier gas is N₂.

In one embodiment, the solvent may be toluene, tetrahydrofuran, aceticacid, acetone, acetonitrile, butanol, dichloromethane, chloroform,chlorobenzene, dichloroethane, diethylene glycol, diethyl ether,dimethoxy-ethane, dimethyl-formamide, dimethyl sulfoxide, ethanol, ethylacetate, ethylene glycol, heptane, hexamethylphosphoramide,hexamethylphosphorous triamide, methanol, methyl t-butyl ether,methylene chloride, pentane, cyclopentane, hexane, cyclohexane, benzene,dioxane, propanol, isopropyl alcohol, pyridine, triethyl amine,propandiol-1,2-carbonate, ethylene carbonate, propylene carbonate,nitrobenzene, formamide, γ-butyrolactone, benzyl alcohol,n-methyl-2-pyrrolidone, acetophenone, benzonitrile, valeronitrile,3-methoxy propionitrile, dimethyl sulfate, aniline, n-methylformamide,phenol, 1,2-dichlorobenzene, tri-n-butyl phosphate, ethylene sulfate,benzenethiol, dimethyl acetamide, N,N-dimethylethaneamide,3-methoxypropionnitrile, diglyme, cyclohexanol, bromobenzene,cyclohexanone, anisole, diethylformamide, 1-hexanethiol, ethylchloroacetate, 1-dodecanthiol, di-n-butylether, dibutyl ether, aceticanhydride, m-xylene, o-xylene, p-xylene, morpholine, diisopropyletheramine, diethyl carbonate, 1-pentandiol, n-butyl acetate, and/or1-hexadecanthiol. In one embodiment, the solvent comprises pyridine,N,N-dimethylformamide (DMF), N,N-dimethylacetamide, N-methyl pyrrolidone(NMP), hexamethylphosphoramide (HMPA), dimethyl sulfoxide (DMSO),acetonitrile, tetrahydrofuran (THF), 1,4-dioxane, dichloromethane,chloroform, carbon tetrachloride, dichloroethane, acetone, ethylacetate, pentane, hexane, decalin, dioxane, benzene, toluene, xylene,o-dichlorobenzene, diethyl ether, methyl t-butyl ether, methanol,ethanol, ethylene glycol, isopropanol, propanol, and/or n-butanol. In apreferred embodiment, the solvent is acetone, methanol, ethanol, and/orisopropanol. More preferably the solvent is methanol, and in anotherembodiment, the solvent consists essentially of methanol.

In one embodiment, the solvent may comprise water. The water used as asolvent or for other purposes may be tap water, distilled water,bidistilled water, deionized water, deionized distilled water, reverseosmosis water, and/or some other water. In one embodiment the water isbidistilled or treated with reverse osmosis to eliminate trace metals.Preferably the water is bidistilled, deionized, deionized distilled, orreverse osmosis water, and at 25° C. has a conductivity of less than 10μS·cm⁻¹, preferably less than 1 μS·cm⁻¹; a resistivity of greater than0.1 MA cm, preferably greater than 1 MA cm, more preferably greater than10 MA cm; a total solid concentration of less than 5 mg/kg, preferablyless than 1 mg/kg; and a total organic carbon concentration of less than1000 μg/L, preferably less than 200 μg/L, more preferably less than 50μg/L.

Preferably the solvent and the cobalt complex and/or vanadium complexare able to form an appropriately soluble solution that can be dispersedin the carrier gas as aerosol particles. For instance, the cobaltcomplex and/or vanadium complex may first be dissolved in a volume ofsolvent, and then pumped through a jet nozzle in order to create anaerosol mist. In other embodiments, the mist may be generated by apiezoelectric ultrasonic generator. Other nebulizers and nebulizerarrangements may also be used, such as concentric nebulizers, cross-flownebulizers, entrained nebulizers, V-groove nebulizers, parallel pathnebulizers, enhanced parallel path nebulizers, flow blurring nebulizers,and piezoelectric vibrating mesh nebulizers. In one embodiment, themixtures of the cobalt complex and solvent, and the vanadium complex andsolvent, are introduced as separate aerosols, for instance, produced byseparate nozzles or nebulizers. Preferably, however, the cobalt complexand vanadium complex are mixed together in the same solvent prior toproducing the aerosol.

In one embodiment, the aerosol may have a mass concentration M, of 10μg/m³-1,000 mg/m³, preferably 50 μg/m³-1,000 μg/m³. In one embodiment,the aerosol may have a number concentration N, in a range of 10³-10⁶,preferably 10⁴-10⁵ cm³. In other embodiments, the aerosol may have anumber concentration of less than 10³ or greater than 10⁶. The aerosolparticles or droplets may have an equivalent volume diameter (d_(e)) ina range of 20 nm-100 μm, preferably 0.5-70 μm, more preferably 1-50 μm,though in some embodiments, aerosol particles or droplets may have anaverage diameter of smaller than 0.2 μm or larger than 100 μm.

In an alternative embodiment, the oxidation state of the vanadium in thevanadium complex, and/or the cobalt in the cobalt complex, may bereduced or oxidized in the process of the deposition and formation ofthe CoVO_(x) layer. In one embodiment, the aerosol and substrate do notcomprise or contact hydrogen gas or a reducing agent during thecontacting and/or depositing. In a related embodiment, the aerosol andsubstrate do not comprise or contact hydrogen gas or a reducing agentimmediately prior to the contacting and/or depositing. In oneembodiment, the reaction chamber where the depositing takes place isessentially free of hydrogen gas and a reducing agent immediately priorto the contacting. In one embodiment, an intermediate reducing agent iscreated during the contacting.

In a related embodiment, before the contacting and/or depositing, theaerosol consists essentially of the carrier gas, the solvent, thevanadium complex, and the cobalt complex, meaning that at least 99.9 wt%, preferably at least 99.99 wt %, or 100 wt % of the aerosol is carriergas, solvent, vanadium complex, or cobalt complex, relative to a totalweight of the aerosol.

In one embodiment, the aerosol is contacted with the substrate for atime period of 10-30 min, preferably 12-28 min, more preferably 15-25min, even more preferably 17-23 min, or about 20 min.

In one embodiment, during the contacting of the aerosol, the carrier gashas a flow rate in a range of 20-250 cm³/min, preferably 50-230 cm³/min,more preferably 75-200 cm³/min, even more preferably 100-150 cm³/min, orabout 120 cm³/min. Preferably, the aerosol has a flow rate similar tothe carrier gas, with the exception of the portion of aerosol that getsdeposited on the substrate. In one embodiment, the aerosol may enter thechamber and the flow rate may be stopped, so that a portion of aerosolmay settle on the substrate.

In one embodiment, the aerosol is contacted with the substrate in areaction chamber. The flow of the carrier gas and aerosol may have a gashourly space velocity in a range of 10-1,000 h⁻¹, preferably 50-500 h⁻¹,more preferably 100-130 h⁻¹.

The contacting and/or introducing may take place within a closed chamberor reactor, and the aerosol may be generated by dispersing a solution ofthe cobalt complex and/or vanadium complex and solvent. In oneembodiment, this dispersing may be increased by fans, jets, or pumps.However, in another embodiment, an aerosol may be formed in a closedchamber with a substrate where the aerosol particles are allowed todiffuse towards or settle on the substrate. The substrate may have anarea in a range of 0.5-4 cm², preferably 1.0-3 cm², more preferably1.5-3 cm². In one embodiment, the closed chamber or reactor may have alength of 10-100 cm, preferably 12-30 cm, and a diameter or width of1-10 cm, preferably 2-5 cm. In other embodiments, the closed chamber orreactor may have an interior volume of 0.2-100 L, preferably 0.3-25 L,more preferably 0.5-10 L. In one embodiment, the closed chamber orreactor may comprise a cylindrical glass vessel, such as a glass tube.

Being in a closed chamber, the interior pressure of the chamber (andthus the pressure of the aerosol) may be controlled. The pressure may bepractically unlimited, but need not be an underpressure or anoverpressure. Preferably the chamber and substrate are heated and heldat a temperature prior to the contacting, so that the temperature maystabilize. The chamber and substrate may be heated for a time period of5 min-1 hr, preferably 10-20 min prior to the contacting.

During the contacting of the aerosol, the CoVO_(x) layer may form at arate of 0.1 to 20, 0.2 to 18, 0.4 to 16, 0.5 to 14, 0.6 to 12, 0.7 to10, 0.8 to 9, 3 to 15, 1.0 to 8, 1.5 to 5, or 2 to 3 nm/s, and/or atleast 0.01, 0.05, 0.1, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0, 1.5, 1.75, 2, 2.5,3.33, 3.5, 4, 4.5, 5, 6.5, 7, 7.5, 7.75, 8, 8.25, 8.5, 8.75, 9, or 10nm/s. In one embodiment, the layer may form at a rate in a range of0.1-2.0 nm/s, 0.2-1.9 nm/s, 0.3-1.8 nm/s, 0.4-1.7 nm/s, 0.5-1.6 nm/s,0.6-1.5 nm/s, 0.7-1.3 nm/s, or about 0.8 nm/s, or about 1.1 nm/s, orabout 1.3 nm/s.

In one embodiment, the method of making the composite thin filmelectrode may further comprise a step of cooling the composite thin filmelectrode after the contacting. The composite thin film electrode may becooled to a temperature between 10 to 45° C., 20 to 40° C., or 25 to 35°C. under an inert gas (such as N₂ or Ar) over a time period of 0.5 to 5h, 0.75 to 4 h, 1 to 3 h, 1.25 to 2.5 h, or 1.5 to 2 h. In oneembodiment, the composite thin film electrode may be left in the chamberand allowed to cool.

In one embodiment, the method of making the composite thin filmelectrode may further comprise a step of preparing the cobalt complexbefore the contacting. The cobalt complex may be synthesized by methodsdescribed herein, or by mixing Co(OAc)₂, Ti(^(i)Pro)₄, andtrifluoroacetic acid in THF to form a mixture. The mixture may bestirred for 0.5-6 h, preferably 1-3 h under an inert atmosphere of N₂ orAr gas. The reaction mixture may then be dried to yield the cobaltcomplex, or alternatively, the reaction mixture may be dried,resuspended in THF, and then dried a second time to yield the cobaltcomplex.

An example AACVD setup is illustrated in FIG. 10 . Here, a container ofthe Co—V precursor solution 12 (of solvent, cobalt complex, and vanadiumcomplex) is connected to a carrier gas supply 10 and placed in anultrasonic humidifier 14. Aerosol droplets 22 are transferred into areactor tube 20. The reactor tube 20 is positioned in a tube furnace 16with heating zones 18. The aerosol droplets 22 deposit on substrateslides 24 within the reactor tube 20. To support a flow of aerosol, thereactor tube 20 is also connected to a gas trap 26 and an exhaust line28.

In an alternative embodiment, the composite thin film electrode may beformed by drop-drying or immobilizing CoVO_(x) on a conductivesubstrate, such as onto an ITO film or a gold film, or on a carbonsubstrate. In an alternative embodiment, the composite thin filmelectrode, or some other electrode involving nanostructured CoVO_(x),may be formed by lithography, more preferably nanolithography.Nanolithography techniques may be categorized as in series or parallel,mask or maskless/direct-write, top-down or bottom-up, beam or tip-based,resist-based or resist-less methods all of which are acceptable in termsof the present disclosure. Exemplary nanolithography techniques include,but are not limited to, optical lithography, photolithography, directedself-assembly, extreme ultraviolet lithography, electron beamlithography, electron beam direct write lithography, multiple electronbeam lithography, nanoimprint lithography, step-and-flash imprintlithography, multiphoton lithography, scanning probe lithography,dip-pen nanolithography, thermochemical nanolithography, thermalscanning probe lithography, local oxidation nanolithography, molecularself-assembly, stencil lithography, X-ray lithography, laser printing ofsingle nanoparticles, magnetolithography, nanosphere lithography, protonbeam writing, charged particle lithography, ion projection lithography,electron projection lithography, neutral particle lithography andmixtures thereof. In another alternative embodiment, the composite thinfilm electrode may be formed by a sol-gel, solvothermal synthesis, orchemical vapor deposition method. In another alternative embodiment, thecomposite thin film electrode may be synthesized by two or moretechniques, for instance, a nanolithography method and then anelectrodeposition method.

In another alternative embodiment, a layer of CoVO_(x) may be formed asan electrode, and then etched to form a nanostructured surface having anincreased surface area appropriate for electrocatalysis.

According to a third aspect, the present disclosure relates to anelectrochemical cell comprising the composite thin film electrode of thefirst aspect, a counter electrode, and an electrolyte solution incontact with both electrodes. As used herein, the composite thin filmelectrode may be considered the working electrode.

In one embodiment, the electrochemical cell is a vessel having aninternal cavity for holding the electrolyte solution. The vessel may becylindrical, cuboid, frustoconical, spherical, or some other shape. Thevessel walls may comprise a material including, but not limited to,glass, polypropylene, polyvinyl chloride, polyethylene, and/orpolytetrafluoroethylene, and the vessel walls may have a thickness of0.1-3 cm, preferably 0.1-2 cm, more preferably 0.2-1.5 cm. The internalcavity may have a volume of 2 mL-100 mL, preferably 2.5 mL-50 mL, morepreferably 3 mL-20 mL. In another embodiment, for instance, for smallscale or benchtop water oxidation, the internal cavity may have a volumeof 100 mL-50 L, preferably 1 L-20 L, more preferably 2 L-10 L. Inanother embodiment, for instance, for pilot plant water oxidation, theinternal cavity may have a volume of 50 L-10,000 L, preferably 70L-1,000 L, more preferably 80 L-2,000 L. In another embodiment, forinstance, for industrial plant-scale water oxidation, the internalcavity may have a volume of 10,000 L-500,000 L, preferably 20,000L-400,000 L, more preferably 40,000 L-100,000 L. In one embodiment, oneor more electrochemical cells may be connected to each other in paralleland/or in series. In another embodiment, the electrolyte solution may bein contact with more than one working electrode and/or more than onecounter electrode.

In one embodiment, the counter electrode comprises gold, platinum, orcarbon. In a further embodiment, the counter electrode comprisesplatinum. In one embodiment, the counter electrode may be in the form ofa wire, a rod, a cylinder, a tube, a scroll, a sheet, a piece of foil, awoven mesh, a perforated sheet, or a brush. The counter electrode may bepolished in order to reduce surface roughness or may be texturized withgrooves, channels, divots, microstructures, or nanostructures.

In another further embodiment, where the counter electrode comprisesplatinum, the counter electrode is in the form of rod, wire, or a coiledwire. Alternatively, the counter electrode may comprise some otherelectrically-conductive material such as platinum-iridium alloy,iridium, titanium, titanium alloy, stainless steel, gold, cobalt alloyand/or some other electrically-conductive material, where an“electrically-conductive material” as defined here is a substance withan electrical resistivity of at most 10⁻⁶ Ω·m, preferably at most 10⁻⁷Ω·m, more preferably at most 10⁻⁸ Ω·m at a temperature of 20-25° C. Inanother alternative embodiment, the working electrode may not compriseFTO, but may comprise any of the previously mentioned metals.

In a preferred embodiment, the counter electrode has at least one outersurface comprising an essentially inert, electrically conductingchemical substance, such as platinum, gold, or carbon. In anotherembodiment, the counter electrode may comprise solid platinum, gold, orcarbon. The form of the counter electrode may be generally relevant onlyin that it needs to supply sufficient current to the electrolytesolution to support the current required for electrochemical reaction ofinterest. The material of the counter electrode should thus besufficiently inert to withstand the chemical conditions in theelectrolyte solution, such as acidic or basic pH values, withoutsubstantially degrading during the electrochemical reaction. The counterelectrode preferably should not leach out any chemical substance thatinterferes with the electrochemical reaction or might lead toundesirable contamination of either electrode.

In a further embodiment, where the counter electrode comprises platinum,the counter electrode may be in the form of a mesh. In one embodiment,the counter electrode in the form of a mesh may have a nominal apertureor pore diameter of 0.05-0.6 mm, preferably 0.1-0.5 mm, more preferably0.2-0.4 mm, and/or a wire diameter of 0.01-0.5 mm, preferably 0.08-0.4mm, more preferably 0.1-0.3 mm. In other embodiments, the counterelectrode may be considered a gauze with a mesh number of 40-200,preferably 45-150, more preferably 50-100. In other embodiments, thecounter electrode may be in the form of a perforated sheet or a sponge.In one embodiment, the counter electrode may be in the form of a meshwith one or more bulk dimensions (length, width, or thickness) aspreviously described for the composite thin film electrode.

In one embodiment, the counter electrode is in the form of a rod orwire. The rod or wire may have straight sides and a circularcross-section, similar to a cylinder. A ratio of the length of the rodor wire to its width may be 1,500:1-1:1, preferably 500:1-2:1, morepreferably 300:1-3:1, even more preferably 200:1-4:1. The length of therod or wire may be 0.5-50 cm, preferably 1-30 cm, more preferably 3-20cm, and a long wire may be coiled or bent into a shape that allows theentire wire to fit into an electrochemical cell. The diameter of the rodor wire may be 0.5-20 mm, preferably 0.8-8 mm, more preferably 1-3 mm.In one embodiment, the diameter of the rod or wire may be smaller, forinstance, with a diameter in a range of 0.1-1 mm, preferably 0.2-0.5 mm,or about 0.25 mm. In some embodiments, a rod may have an elongatedcross-section, similar to a ribbon or strip of metal.

In one embodiment, the electrolyte solution comprises water and aninorganic base at a concentration of 0.1-1.0 M, preferably 0.2-0.8 M,more preferably 0.3-0.7 M, or about 0.5 M, though in some embodiments,the inorganic base may be present at a concentration of less than 0.1 Mor greater than 1.0 M. For long term electrocatalysis, the organic basemay be present at a concentration in a range of 0.05-0.5 M, preferably0.08-0.2 M, more preferably about 0.1 M. The inorganic base may be KOH,LiOH, NaOH, Be(OH)₂, Mg(OH)₂, Ca(OH)₂, Sr(OH)₂, Ba(OH)₂, or some otherinorganic base. Preferably the inorganic base is KOH. In an alternativeembodiment, an organic base may be used, such as sodium acetate. Inanother alternative embodiment, an acid may be used instead of a base.

The water may be tap water, distilled water, bidistilled water,deionized water, deionized distilled water, reverse osmosis water,and/or some other water. In one embodiment the water is bidistilled toeliminate trace metals. Preferably the water is bidistilled, deionized,deionized distilled, or reverse osmosis water and at 25° C. has aconductivity at less than 10 μS·cm⁻¹, preferably less than 1 μS·cm⁻¹, aresistivity greater than 0.1 MA cm, preferably greater than 1 MA cm,more preferably greater than 10 MA cm, a total solid concentration lessthan 5 mg/kg, preferably less than 1 mg/kg, and a total organic carbonconcentration less than 1000 μg/L, preferably less than 200 μg/L, morepreferably less than 50 μg/L.

In one embodiment, the composite thin film electrode has a currentdensity of 1.0-10 mA/cm², preferably 1.2-9.8 mA/cm², more preferably 2-9mA/cm² when the electrodes are subjected to a bias potential of1.45-1.55 V, preferably 1.47-1.53 V.

In one embodiment, composite thin film electrode has an overpotential ina range of 270-335 mV, preferably 280-325 mV, more preferably 290-320mV, or about 310 mV at a current density of 9-11 mA/cm², 9.5-10.5mA/cm².

Preferably, to maintain uniform concentrations and/or temperatures ofthe electrolyte solution, the electrolyte solution may be stirred oragitated during the step of the subjecting. The stirring or agitatingmay be done intermittently or continuously. This stirring or agitatingmay be by a magnetic stir bar, a stirring rod, an impeller, a shakingplatform, a pump, a sonicator, a gas bubbler, or some other device.Preferably the stirring is done by an impeller or a magnetic stir bar.

In one embodiment, a composite thin film electrode may have a highercurrent density than a bare FTO, where the FTO electrode has essentiallythe same structure without the CoVO_(x) layer. For example, the barecarbon electrode may comprise bare carbonized paper, and may be housedin a similar electrode assembly. Here, over the same range of electricalpotential and in similar electrochemical cells, the composite thin filmelectrode may have a current density that is greater by a factor of3-12, preferably 4-10, than the current density of the bare carbonelectrode. This difference in current densities may lead to thecomposite thin film electrode supporting a faster chemical reaction ratein an electrochemical cell. In one embodiment, a composite thin filmelectrode formed from a shorter deposition time may have a greatercurrent density than another composite thin film electrode formed with alonger deposition time, as illustrated in FIG. 5A. This may be due tothe increased surface area and increased electroactive surface area ofthe composite thin film electrode formed with the shorter depositiontime.

In one embodiment, the electrochemical cell further comprises areference electrode in contact with the electrolyte solution. Areference electrode is an electrode which has a stable and well-knownelectrode potential. The high stability of the electrode potential isusually reached by employing a redox system with constant (buffered orsaturated) concentrations of each relevant species of the redoxreaction. A reference electrode may enable a potentiostat to deliver astable voltage to the working electrode or the counter electrode. Thereference electrode may be a standard hydrogen electrode (SHE), a normalhydrogen electrode (NHE), a reversible hydrogen electrode (RHE), asaturated calomel electrode (SCE), a copper-copper(II) sulfate electrode(CSE), a silver chloride electrode (Ag/AgCl), a pH-electrode, apalladium-hydrogen electrode, a dynamic hydrogen electrode (DHE), amercury-mercurous sulfate electrode, or some other type of electrode. Ina preferred embodiment, a reference electrode is present and is a silverchloride electrode (Ag/AgCl), while for long term electrocatalysis, asaturated calomel electrode (Hg/HgO) was used. However, in someembodiments, the electrochemical cell does not comprise a thirdelectrode.

According to a fourth aspect, the present disclosure relates to a methodfor decomposing water into H₂ and O₂. This method involves the step ofsubjecting the electrodes of the electrochemical cell of the thirdaspect with a potential of 0.5-2.0 V, preferably 0.6-1.8 V, morepreferably 0.8-1.7 V. Here, “the electrodes” refers to the compositethin film electrode and the counter electrode. However, in someembodiments, the electrodes may be subjected to a potential of less than0.5 V or greater than 2.0 V.

Preferably the composite thin film electrode functions as the anode,receiving a positive potential to oxidize OH⁻ into O₂ gas and H₂O, whilethe counter electrode functions as the cathode, receiving a negativepotential to reduce water into H₂ gas and OH⁻. This is summarized by thefollowing reactions:

Cathode (reduction): 2H₂O_((l))+2e⁻→H_(2(g))+2OH⁻ _((aq))

Anode (oxidation): 4OH⁻ _((aq))→O_(2(g))+2H₂O_((l))+4e⁻

Overall reaction: 2H₂O_((l))→2H_(2(g))+O_(2(g))

In another embodiment, the potentials may be switched, wherein thecomposite thin film electrode functions as the cathode and receives anegative potential, and the counter electrode functions as the anode andreceives a positive potential. In an alternative embodiment, theelectrodes may be subjected to an alternating current (AC) in which theanode and cathode roles are continually switched between the twoelectrodes.

In one embodiment, the potential may be applied to the electrodes by abattery, such as a battery comprising one or more electrochemical cellsof alkaline, lithium, lithium-ion, nickel-cadmium, nickel metal hydride,zinc-air, silver oxide, and/or carbon-zinc. In another embodiment, thepotential may be applied through a potentiostat or some other source ofdirect current, such as a photovoltaic cell. In one embodiment, apotentiostat may be powered by an AC adaptor, which is plugged into astandard building or home electric utility line. In one embodiment, thepotentiostat may connect with a reference electrode in the electrolytesolution. Preferably the potentiostat is able to supply a relativelystable voltage or potential. For example, in one embodiment, theelectrochemical cell is subjected to a voltage that does not vary bymore than 5%, preferably by no more than 3%, preferably by no more than1.5% of an average value throughout the subjecting. In anotherembodiment, the voltage may be modulated, such as being increased ordecreased linearly, being applied as pulses, or being applied with analternating current. Preferably, the composite thin film electrode maybe considered the working electrode with the counter electrode beingconsidered the auxiliary electrode. However, in some embodiments, thecomposite thin film electrode may be considered the auxiliary electrodewith the counter electrode being considered the working electrode.

In one embodiment, the composite thin film electrode has a mass activityin range of 38-50 A/g, preferably 40-48 A/g, more preferably 42-46 A/gat η=350 mV. The specific potential value may be 1.45-1.60 V, morepreferably 1.48-1.58 V, or 1.58 V vs. RHE.

In one embodiment, the method also involves the step of separatelycollecting H₂-enriched gas and O₂-enriched gas. In one embodiment, thespace above each electrode may be confined to a vessel in order toreceive or store the evolved gases from one or both electrodes. Thecollected gas may be further processed, filtered, or compressed.Preferably the H₂-enriched gas is collected above the cathode, and theO₂-enriched gas is collected above the anode. The electrochemical cell,or an attachment, may be shaped so that the headspace above thecomposite thin film electrode is kept separate from the headspace abovethe reference electrode. In one embodiment, the H₂-enriched gas and theO₂-enriched gas are not 100 vol % H₂ and 100 vol % O₂, respectively. Forexample, the enriched gases may also comprise N₂ from air, and watervapor and other dissolved gases from the electrolyte solution. TheH₂-enriched gas may also comprise O₂ from air. The H₂-enriched gas maycomprise greater than 20 vol % H₂, preferably greater than 40 vol % H₂,more preferably greater than 60 vol % H₂, even more preferably greaterthan 80 vol % H₂, relative to a total volume of the receptaclecollecting the evolved H₂ gas. The O₂-enriched gas may comprise greaterthan 20 vol % O₂, preferably greater than 40 vol % O₂, more preferablygreater than 60 vol % O₂, even more preferably greater than 80 vol % O₂,relative to a total volume of the receptacle collecting the evolved O₂gas. In some embodiments, the evolved gases may be bubbled into a vesselcomprising water or some other liquid, and higher concentrations of O₂or H₂ may be collected. In one embodiment, evolved O₂ and H₂, orH₂-enriched gas and O₂-enriched gas, may be collected in the samevessel.

Several parameters for the method for decomposing water may be modifiedto lead to different reaction rates, yields, and other outcomes. Theseparameters include, but are not limited to, electrolyte type andconcentration, pH, pressure, solution temperature, current, voltage,stirring rate, electrode surface area, texture and nanostructure of theCoVO_(x) layer, substrate conductivity, and exposure time. A variable DCcurrent may be applied at a fixed voltage, or a fixed DC current may beapplied at a variable voltage. In some instances, AC current or pulsedcurrent may be used. A person having ordinary skill in the art may beable to adjust these and other parameters, to achieve different desirednanostructures. In other embodiments, the electrochemical cell may beused for other electrochemical reactions or analyses.

In an alternative embodiment, the composite thin film electrode may beused in the field of batteries, fuel cells, photochemical cells, watersplitting cells, electronics, water purification, hydrogen sensors,semiconductors (such as field effect transistors), magneticsemiconductors, capacitors, data storage devices, biosensors (such asredox protein sensors), photovoltaics, liquid crystal screens, plasmascreens, touch screens, OLEDs, antistatic deposits, optical coatings,reflective coverings, anti-reflection coatings, and/or reactioncatalysis. Similarly, in one embodiment, the composite thin filmelectrode may be coated with another material. For example, thecomposite thin film electrode may be coated with a layer of gold. Agold-coated composite thin film electrode may then be used for analytedetection using surface enhanced Raman scattering (SERS).

The examples below are intended to further illustrate protocols forpreparing, characterizing the CoVO_(x) films, and uses thereof, and arenot intended to limit the scope of the claims.

Example 1 Film Electrode Fabrication

The film electrode fabrication was achieved through AACVD method. Allchemicals were obtained from Sigma Aldrich and were used as received:Cobalt(II) acetylacetonate (Co(acac))₂), vanadium(III) acetylacetonate(V(acac)₃), methanol (99.9%), and nitrogen gas (99.99%). The synthesisof Co:V oxide films in a 1:1 stoichiometry was achieved by dissolving500 mg, (0.19 mmol) of Co(acac)₂ and 500 mg (0.14 mmol) V(acac)₃ inmethanol (20 mL) in a schlenk tube connected with a vacuum line. Thetransparent dark brown solution was stirred for 30 min and solvent wasevaporated under reduced pressure to give a brown solid which wasre-dissolved in methanol (10 mL). The transparent solution was furtherstirred for 10 min and was used in AACVD for films deposition. Prior tothe deposition, an FTO glass substrate was cut to an area of 1.0×2.0 cm²(W×L) and sequentially washed using soapy water, acetone, and ethanol.The substrate was then laid horizontally inside the reactor tube andheated up to the deposition temperature of 475° C. for 10 min tostabilize the temperature before carrying out the deposition. An aerosolmist of the precursor solution was generated using a piezoelectricultrasonic humidifier. Nitrogen gas was used as a carrier gas totransport the aerosol to the heated substrate at a rate of 120 cm³/min.The reactor exhaust was vented into a fume hood. When the precursorsolution and associated aerosol mist had been completely emptied fromthe flask, the coated substrate was cooled under a continuous flow of N₂gas. The coated substrate was not removed from the reactor until itreached a temperature of below 40° C.

The deposition experiments were carried out for different time periodssuch as 20 min, 40 min, and 60 min, and the resultant film electrodesare named as CoVO_(x)-20, CoVO_(x)-40, and CoVO_(x)-60, respectively.

Example 2 Film Characterization

The structural properties of cobalt vanadium oxide thin films wereanalyzed by recording X-ray diffraction (XRD) patterns using aPANanalytical, X'PertHighScore® diffractometer with primarymonochromatic high intensity CuK_(α) (λ=1.5418 Å) radiation. The surfacemicrographs of the films were examined using a Lyra 3® Tescan, fieldemission gun (FEG)-SEM at an accelerating voltage of 5 kV and a workingdistance of 10 mm. The Co/V atomic ratios were determined from Energydispersive X-ray (EDX, INCA Energy 200®, Oxford Inst.) spectrometer.X-ray photoelectron spectroscopy (XPS) was done using an ULVAC-PHIQuantera II® with a 32-channel Spherical Capacitor Energy Analyzer undervacuum (1×10⁻⁶ Pa) using monochromatic Al Kα radiation (1486.8 eV) witha natural energy width of 680 meV. The carbonaceous C is line (284.6 eV)was used as a reference to calibrate the binding energies.

Example 3 Electrochemical Measurements

All the electrochemical measurements were performed on acomputer-controlled AUTOLAB® potentiostat employing CoVO_(x) thin filmelectrodes as the working electrode. A Pt wire shaped into a spiral(thickness=0.25 mm) was used as the counter electrode and saturatedsilver-silver chloride (Ag/AgCl in saturated solution of KCl) was usedas the reference electrode. For long-term electrocatalysis, a saturatedcalomel electrode (Hg/HgO) reference electrode was employed in 0.1 M KOHsolutions. However, all the potentials are referred to reversiblehydrogen electrode (RHE) following the Nernst equation:

E_(RHE)=E_(REF)+E_(0 REF)+0.059(pH).

Before placing into the electrochemical cell, the platinum wire wascleaned by immersing in a 20% solution of HNO₃ for a few minutesfollowing washing with MilliQ® water. All the glassware andelectrochemical cell were cleaned by boiling in a 1:3 mixture of H₂SO₄and HNO₃ followed by boiling in water. The electrochemical cell was thencarefully rinsed with acetone and dried by keeping in oven at 100° C.for 1 hour as described previously. See Yu, F.; Li, F.; Zhang, B.; Li,H.; Sun, L., Efficient Electrocatalytic Water Oxidation by a CopperOxide Thin Film in Borate Buffer. ACS Catalysis 2015, 5 (2), 627-630,incorporated herein by reference in its entirety. Electrochemicalinvestigations such as cyclic voltammetry, EIS and controlled potentialbulk electrolysis experiments were performed in 0.5 M KOH electrolytesolution having a pH≈13.6. Water used to make all the solutions forelectrochemical studies was distilled and deionized using a MilliQ®system from Millipore. Linear sweep voltammetry was used in order tofind the overpotentials and current density profiles of the films duringwater oxidation reaction whereas the charge transfer resistances weredetermined by EIS. The details of measurement parameters such as thecalculation of mass activity and the electrochemically active surfacearea (ECSA) are provided herein.

Example 4

Scheme of Synthesis with Structural and Morphological Analysis:

The schematic description for the fabrication of mixed metal oxide filmsof cobalt and vanadium is provided in FIG. 10 . The fabrication wasperformed using an AACVD protocol, the operation of which is criticallyrelated to the solubility of the precursors in common organic solvents.Moreover, the precursor solutions must be homogenous, clear, andprecipitation free, especially in the case of mixed metal oxides.Therefore, for the fabrication of Co—V films, acetylacetonate precursorsof both metals were chosen, which are commercially available and knownfor their higher solubility in methanol without using any solubilityenhancing reagents such as trifluoroacetic acid. With this selection ofsame ligand system for both metals, the possibility of exchangereactions was ruled out, which may cause solution inhomogeneity with thepassage of time. This results in a superior particle-particle orparticle-conducting layer connection during direct deposition to forcewell adhered films, possibly on a variety of available substrates. Thedeposition of these films was done directly on the FTO electrodes at arelatively low temperature of 475° C. without any need of furtherimmobilization as in the case of hydrothermal synthesis, colloidalapproaches, or other wet chemistry protocols. See Tan, C.; Zhang, H.,Wet-chemical synthesis and applications of non-layer structuredtwo-dimensional nanomaterials. Nature Communications 2015, 6, 7873,incorporated herein by reference in its entirety. Three different filmswere prepared at deposition times of 20, 40, and 60 minutes, and thistime is multiple orders lesser than wet chemistry protocols oftenneeding 24-48 hours for the reaction to complete. These films werecorrespondingly named CoVO_(x)-20, CoVO_(x)-40, and CoVO_(x)-60.

Surface morphology and the corresponding cross sectional images of theprepared films were investigated by FE-SEM, and the data is provided inFIGS. 2A-2I. Large area analysis of all the films (FIGS. 2A, 2D, 2G)shows a uniform and homogenous surface character, even for the extendeddeposition time up to 60 min. However, the morphology of the surfacebecomes more varied as the time of deposition increases. It is clearlyseen that CoVO_(x)-20 film attains a spongy character with growth in alldimensions (FIG. 2A), which appeared to be a network of interwovennanofibers stacked over each other at a higher resolution scan (FIG.2B). The pore size in this case was found to be 3-4 nm with extremelyhomogenous distribution. Such a porous and high surface area structureprovides a higher access of the active sites to the reacting substrates,which is extremely useful for the electrocatalytic reactions. InCoVO_(x)-40 film, the networked surface structure has transformed intonanoflakes (FIG. 2E) which, with further growth in case of CoVO_(x)-60,has changed into a thick continuous film with flakes protruding out ofit (FIG. 2H). However, the imprints of networked CoVO_(x)-20 structureremained visible in other two films, showing a coalesced growth. It isobserved that the porous womb-like structure self-propagated into densethick layer of CoVO_(x). The cross sectional image of CoVO_(x)-20 showsthat a 1 μm thick film (FIG. 2C) has already been fabricated in just 20min, which was then grown into 3 μm and 4 μm thickness for CoVO_(x)-40(FIG. 2F) and CoVO_(x)-60 (FIG. 2I) respectively. However, this growthpattern has not disrupted the homogeneity of the formed films as shownby continuous thickness of all the cross sections.

FIG. 11 demonstrates the XRD analysis of all CoVO_(x) films prepared onFTO substrates at 475° C. The diffractogram reveals the amorphous natureof all the films even for the extended deposition time of 60 min. Whilethe FTO substrate is highly crystalline in nature, its crystalline peaksare suppressed due to the non-crystalline profile of the preparedmaterials in all cases. To confirm the amorphous nature of CoVO_(x), thepristine films of CoO_(x) and VO_(x) were also prepared under similarAACVD conditions using the individual precursors (i.e., Co(acac)₂ andV(acac)₃) and their diffraction patterns are shown in FIGS. 12A and 12B.These patterns show crystalline peaks of individual CoO_(x) and VO_(x)in pure form. It is an indication that the mixed films thus prepared arefree of crystalline impurities of Co oxide and V oxide and that only themixed material is amorphous in nature. At the same time, this is quitein accordance with the previous reports of amorphous Co—V mixed oxidematerials prepared by hydrothermal methods and co-precipitationtechnique, although the materials were synthesized at more extremeconditions of pressure or temperature. See Liardet et al. (2018); andLiu et al. (2018). Thus, it may be concluded that the inherent nature ofmixed Co—V oxides is to be amorphous, although the synthetic schemes inprevious reports claim otherwise.

The composition and elemental stoichiometry of all the films werecharacterized by energy dispersive x-ray (EDX) analysis, and theresulting spectra are shown in FIGS. 3A, 3D, and 3G for the samplesCoVO_(x)-20, CoVO_(x)-40, and CoVO_(x)-60, respectively, with respectivespectra taken from the regions shown in FIGS. 3C, 3F, and 3I. FIGS. 3B,3E, and 3F also show the values of percent atomicity for both metals inthe films CoVO_(x)-20, CoVO_(x)-40, and CoVO_(x)-60, respectively. TheCo:V ratio found in each case is nearly equal to unity. These data areprovided in tabulated form in Table 1. Further, the presence of bothcobalt and vanadium elements in the film was ascertained by conductingthe EDX mapping. FIGS. 13A-13I indicate the uniform distribution of Coand V elements in all the films. FIGS. 13A, 13D, and 13G show FESEMimages of CoVO_(x)-20, CoVO_(x)-40, and CoVO_(x)-60, respectively. FIGS.13B, 13E, and 13H show the VKa1 signal obtained from the regions ofFIGS. 13A, 13D, and 13G, respectively. FIGS. 13C, 13F, and 13I show theCoKa1 signal obtained from the regions of FIGS. 13A, 13D, and 13G,respectively.

TABLE 1 Atomicities of the two metals and their atomic ratio in theresulting film. % atomicity % atomicity Sample Co V Co:V CoVOx-20 9.1010.58 0.86:1 CoVOx-40 16.62 15.15  1.1:1 CoVOx-60 20.24 19.53 1.05:1

The amorphous CoVO_(x)-20 film was further characterized by X-rayphotoelectron spectroscopy (XPS) in order to find out the oxidationstates of the constituent elements. The survey spectrum so obtainedindicated the presence of Co 2p, V 2p and O 1s in the film. The atomicratio of cobalt to vanadium (Co:V) determined by XPS is approximately16.56:15.95 (1:1) and is consistent with the metallic ratio of the bothelements from the EDX analysis. The high resolution deconvoluted spectraof these individual elements are provided in FIGS. 4A-4C. The bindingenergy 781.1 eV of Co 2p spectra can be fitted to Co⁺² (FIG. 4A).Moreover, two satellite peaks are indicated at binding energies of 786and 788 eV, which are characteristic of high-spin Co²⁺. Contrarily, V 2pspectrum has a peak observed at 2p 3/2 (517.5 eV) which indicates theV⁺⁴ oxidation state (FIG. 4B). Furthermore, the unsymmetrical peak O 1scan be further split into three peaks. The high resolution signals atbinding energies of 531.5 eV and 530.0 eV in the O 1s spectrum representoxygen atoms in hydroxyl group and oxide group, respectively. Thisdemonstrates a characteristic feature for O that is bonded to a metal inmetal oxides. The binding energy peak at 532.3 eV is again a feature ofoxygen atoms in carbonate group (FIG. 4C). Although the XRD studiescould not exactly reveal the chemical formula of the deposited materialdue to the non-crystalline nature of the fabricated material, on thebasis of XPS studies the oxidation state of Co and V atoms existing inthe binary oxide system can be demonstrated. These XPS studies also showa good agreement with the CoVO_(x) materials fabricated by othersynthetic strategies. See Liardet et al. (2018); and Thorat et al.(2018).

Example 5 Electrochemical Water Oxidation Studies

Directly deposited amorphous catalytic films of CoVO_(x) on the FTOsubstrates were used for water oxidation studies without any furthermodification. FTO substrates are much less conductive than the reportedglassy carbon and nickel foam, carbon foam, and gold foam materials usedfor CoVO_(x) immobilization, but are less costly, easily available, andscalable for large area applications. See Liardet et al. (2018); Thoratet al. (2018); and Liu et al. (2018). The water oxidation experimentswere performed in 0.5 M KOH using three electrode configuration underforward potentials sweeps, and the linear sweeps voltammetry (LSV)profiles were obtained. FIG. 5A indicates these profiles for all threeCo—V films at a scan rate of 10 mV/sec. All the data was compared at acurrent density of 10 mA/cm², which is often considered as a referencefor providing 10% efficiency in water splitting reactions. Here,CoVO_(x)-20 film showed remarkable performance in terms of onsetoverpotential (i.e., 270 mV), overpotential at 10 mA/cm² (i.e., 310 mV),and current density reaching to a value of 160 mA/cm² only at anoverpotential of 410 mV. Ibis catalytic performance of CoVO_(x)-20 iseven higher compared to the other two films which both show a similaronset potential but have different overpotential at a current density of10 mA/cm². Better performance of CoVO_(x)-20 can be justified on thebasis of porous nanofiber film structure indicated by the SEM topography(FIGS. 2A and 2B). This porous structure with spongy appearancefacilitated the higher number of catalytic sites to be accessible forthe reaction to proceed. As a consequence, the overpotential is reduced,and the current density jumps to higher values at lower potentials. Withan increase in deposition time, the porosity of the film startsdeteriorating, and the surface structure becomes more compact, asevident from FIG. 2E. Thus, less porous flakes of the material areformed, with an increase in overpotential (i.e., 350 mV and 369 mV forCoVO_(x)-40 and CoVO_(x)-60, respectively as shown in FIG. 5B. Thecurrent densities of these films were also shifted towards lower values.The performance of the CoVO_(x)-20 film was also compared to its formingmaterials and to the film structures reported using other fabricationstrategies. The overpotential for CoVO_(x)-20 film is much smaller thanthe reported values of individual metal oxides of the combination and islower than many cobalt vanadium oxide catalysts, such as Co₂V₂O₇ (340mV), Co₃V₂O₈ (359 mV), and Co₃V₂O₈ nanoroses (391 mV). See Peng et al.(2017); and Xing, M.; Kong, L.-B.; Liu, M.-C.; Liu, L.-Y.; Kang, L.;Luo, Y.-C., Cobalt vanadate as highly active, stable, noble metal-freeoxygen evolution electrocatalyst. Journal of Materials Chemistry A 2014,2 (43), 18435-18443, each incorporated herein by reference in theirentirety. This value is also comparable to the values reported foramorphous cobalt vanadium oxide on glassy carbon (330 mV), nickel foam(260 mV), and gold foam (215 mV), although the substrates used hereinare FTO and are much less conductive than metallic foams. See Liardet etal. (2018); and Liu et al. (2018), as cited previously. A notably highercurrent density is also achieved without a substantial increase ofoverpotential, which makes this AACVD strategy more direct and viablefor catalyst production.

An effect of change in the scan rate on the CoVO_(x)-20 film was alsostudied in FIGS. 6A and 6B. It was found that increasing the scan ratefrom 1 mV/sec to 100 mV/sec shifted the onset potential to more negativepotential values. However, there is no substantial change in theoverpotential for 10 mA/cm² current density as this value shifted from310 mV at a scan rate of 10 mV/sec to 308 mV at a scan rate of 1.0mV/sec, as shown in FIG. 6B. Notable here is that the current densitiesare higher at low scan rates, reaching a value of 175 mA/cm² for 1mV/sec at an overpotential of just 380 mV. With a direct and rapidsynthetic strategy taking only minutes for the whole preparationprocess, and in view of the less conductive behavior of the FTOsubstrates, this water splitting performance is quite remarkable.

In order to study the sustainability and the consistent rate of wateroxidation reaction, Tafel plots were drawn for all the prepared catalystfilms within the linear regions of the current voltage curves at a scanrate of 10 mV/sec and were fitted into the Tafel equation. Ibis analysisprovides an indication of whether the catalyst can operate over a narrowpotential range while producing high current density. A small Tafelslope is an expression of well-balanced kinetics during catalysis. SeeShinagawa, T.; Garcia-Esparza, A. T.; Takanabe, K., Insight on Tafelslopes from a microkinetic analysis of aqueous electrocatalysis forenergy conversion. Scientific Reports 2015, 5, 13801, incorporatedherein by reference in its entirety. These plots with their slope valuesare shown in FIG. 7 . All three catalytic films presented heredemonstrated the enhanced kinetics while displaying appreciably lowvalue of Tafel slopes, however, the film formed with 20 min depositionhas a very large linear range with a low slope of 75 mV/dec. TheCoVO_(x)-40 film also showed a similar linear range but with a highervalue of the slope (i.e., 84 mV/dec). CoVO_(x)-60 film, on the otherhand, staredt deviating from the linear range at log value of 1.4 with aslope of 149 mV/dec. The Tafel slope value for the CoVO_(x)-20 at a scanrate of 1.0 mV/sec was even shifted to 62 mV/dec with a linearity ofresponse even beyond a logarithmic value of 2.2. This indicates thatcatalytic performance is dependent upon the porosity and surfacestructure of the material, and so is the kinetics of the reaction whichis more visible at lower scan rates. An open structure and porousmorphological features of the CoVO_(x)-20 film catalyst supports thefast mass transfer and boosts the electron transfer without undergoingany scattering losses, as a higher number of accessible catalytic sitesare readily available.

For a quantitative analysis of the film properties, mass activities ofthe catalytic films were determined at 1.55 V vs RHE as provided inTable 2. This data also corroborates the Tafel plots where the smallerTafel slopes are linked to the higher mass activity in the same order.The higher mass activity of the CoVO_(x)-20 (i.e., 43.4 A/g) as comparedto the other two films further demonstrates the high mass performance of20 min deposition of the material. Here, a longer deposition and highermass loading does not contribute to the catalytic activity, rather themicrostructure of the film and the available catalytic sites areresponsible for the OER. For an estimation of the available catalyticsites, measurement of the active surface area can be regarded as animportant factor which was numerically assessed from the double-layercapacitances measurements. For that purpose, charging current densitydifferences in a potential window of non-faradaic region were plottedagainst scan rates in FIG. 14 , and the slopes were divided by theelectrode area to get estimated ECSA values in units of mF/cm². Asignificantly higher linear slope value of CoVO_(x)-20 film (i.e., 16.87mF/cm²) as compared to CoVO_(x)-40, which has a value of 10.12 mF/cm²,and CoVO_(x)-60 which has a value of 7.06 mF/cm², indicates that theCoVO_(x)-20 film has a higher number of active sites available, therebymaking the catalytic reaction kinetically favorable.

TABLE 2 Summary of electrocatalytic activity for Co-V mixed oxide thinfilm electrocatalysts. Current Density at η@100 1.60 mV vs Tafel Massη@10 mA/cm² mA/cm² RHE Slope Activity ECSA Sample [mV] [mV] [mA/cm²] [mV/dec] [A/g] [mF/cm²] CoVOx-20 310 370 48  75 43.4 17.63 CoVOx-40 350480 25  84 35.9 10.12 CoVOx-60 369 590 13 134 29.2  7.06

The conductivities of the sample films were estimated using impedancespectroscopy, showing a characteristic depressed semicircle of an OERcharge transfer reaction for all the films. However, it was found thatCoVO_(x)-20 film exhibits highest conductivity which is an indication ofits lowest charge-transfer resistance compared to other two films asdetailed in FIG. 15 . FIG. 15 shows a Nyquist plot for CoVO_(x)-20,CoVO_(x)-40, and CoVO_(x)-60 films at an applied potential of 1.48 V vs.RHE in the frequency range of 0.1 Hz to 100 KHz. For each EIS analysis,data were fitted employing Randles circuit with Nova software. If thisresistance is compared to the materials immobilized on gold foam (i.e.,10-25 ohm), nickel foam (i.e., 1.0-1.4 ohm), glassy carbon (i.e, 0.6-0.8ohm), and carbon foam (4.0-8.0 ohm), it is at least ten times higherbecause of direct deposition on FTO substrate. See Liu et al. (2018);Liardet et al. (2018); and Thorat et al. (2018). Thus, the method ofAACVD deposition can be regarded as highly efficient for water oxidationchemistry. Among the three AACVD films, the higher conductivity of theCoVO_(x)-20 may have originated from its thinner structure with thenetwork of fibers having an enhanced number of active sites available.The larger number of catalytic sites transforms more and more of thecobalt species into their active form, in real time during thecatalysis, thus promoting the charge transfer process. Consequently, theconductivity as well as the catalytic performance of this film issignificantly improved.

In addition to the enhanced catalytic activity with low conductingsubstrates, the CoVO_(x) films also exhibited highly desirable catalyticdurability and stability. For more than 5 h of constant anodicpolarization in each case of 20 mA/cm² and 100 mA/cm² current densities,only a moderate increase of 10-30 mV in overpotential was required forall three fabricated films, as shown in FIG. 8A. This nominal shift inthe overpotentials is caused by the accumulation of very high density ofoxygen covering the active sites at the electrode surface. Bubbles inthe form of a rich continuous stream of oxygen bubbles was also visibleduring the electrocatalytic experiments owing to the high rate of oxygenproduction at these electrodes. Moreover, the CoVO_(x)-20 electrodeexhibited only a small loss of activity after 500 CV cycles as shown inFIG. 8B. All these performance parameters of the fabricated films canonly be attributed to their excellent catalytic activity under theemployed conditions.

Example 6 Calculation of Different Electrocatalytic ParametersElectrochemical Impedance Spectroscopy

EIS analysis was carried out to get more insight into electrochemicalkinetics for all the thin film electrocatalysts. The data was recordedat an applied potential of 1.49 V vs. RHE considering the faradaicregion of cyclic voltammogram to investigate charge transfer resistanceat the so-called electrode-electrolyte double layer.

Mass Activity (MA) [mA·mg⁻¹]

The loading normalized current density or mass activity is calculatedaccording to the following formula:

${MA} = {\frac{{J@\eta} = {0.35V}}{{Active}{mass}{of}{catalyst}}.}$

Here, J is current density in mA·cm⁻² at specific potential value. 1.58V vs. RHE was chosen as the specific potential value.Electrochemically Active Surface Area (ECSA) [mF/cm²]

Electrochemically active surface area (ECSA) was calculated using CVmode by calculating double layer capacitance employing the followingformula:

${ECSA} = \frac{CDL}{Cs}$

First of all, the non-faradaic region (somewhere in between the oxygenand hydrogen region) in the CV was identified by visual analysis ofcyclic voltammetry data assuming that all the current in this potentialrange was due to the double layer charging. Under this potential rangethe CV was run at different scan rates (5 mV·s⁻¹, 10 mV·s⁻¹, 20 mV·s⁻¹,50 mV·s⁻¹). The charging current (Ic) is calculated by identifying amiddle potential range, which was 0.955 V vs. RHE, and the currentassociated with this potential range was considered as capacitivecurrent or charging current. A plot of scan rate versus capacitivecurrent was constructed, and the slope of this calibration curve gave avalue of double layer capacitance per unit area, which serves as theestimation of ECSA.

Example 7 Proposed Mechanism

All of the above measurements as well as related works have led to aproposed mechanism of the catalytic process and the role of vanadiummoieties in the system. See Ehsan et al. (2018); Liu et al. (2018); andXing et al. (2018), as cited previously and incorporated herein byreference in their entirety. As shown in FIG. 9 , whether V is presentor absent, the first few steps of the process are same which involvesthe activation of Co sites in the alkaline medium. The reaction maybegin with the adsorption of water and discharge of hydrogen andelectrons to form adsorbed hydroxyl groups on the surface. Thesehydroxyl groups may react with more hydroxyl ions under the forwardpotential sweep, thereby leaving the oxygen atoms adsorbed onto the filmsubstrate. In the absence of V, the reaction continues with thegeneration of OOH groups at the surface, which is considered as the ratelimiting step. Then in the next step, a water molecule leaves thesurface leaving an oxygen molecule adsorbed on the surface. This oxygenis then removed, completing the oxygen evolution reaction in the laststep. However, in the presence of V, which is capable of switching itsground state, thereby modifying metal-metal and antibonding interactionsin the catalytic cluster, the adsorbed oxygen species on the surface canform oxobridged entities among the neighboring oxygen atoms. This schemeof operation benefits the catalytic reaction in two ways. First, thisoxobridged state is relatively more active, which leads to athermodynamically favorable generation of a hydroperoxo intermediate andkinetically faster O—O bond formation, although it still remains a ratelimiting step. Second, the oxobridged entities can also act as acatalyst, facilitating the reactions forming adsorbed hydroxyl andisolated oxygen species on the catalytic site. Thus, a favorablecatalytic cycle is achieved leading to better performance of mixed oxidefilms. The presence of V in Co-oxide materials has also been shown tohave a decreased overpotential at Co-active sites adjacent to V.However, the V-active sites show increased overpotential. Thiscorresponds exactly to the volcano plots described earlier which showthat a mixed oxide configuration can stabilize the bond strength anddecrease the overpotential. This enhanced catalytic impact of V on theneighboring Co atoms can be ascribed to the coordination environment ofCo atom and its modification caused by the lattice mismatch when V atomsare also present close by. In this manner, the Co-active sites canattain favorable water oxidation energies and the enhanced activity ofOER catalysis is achieved.

Example 8 Observations

Amorphous CoVO_(x) films fabricated herein have shown a highly efficientcatalytic character. Further, the target of obtaining well adhered anduniform films of all the materials was achieved directly on thesubstrate surfaces with changing morphological and catalytic characterwith variations in deposition times. The CoVO_(x)-20 film, due to itspeculiar networked structure exposing larger number of catalytic sites,its high mass activity, larger ECSA, and low charge transfer resistancedemonstrated lower overpotential, higher current density, and lowerTafel slope as compared to CoVO_(x)-40 and CoVO_(x)-60. These numberswere comparable to only a few CoVO_(x) materials previously reported,however, a clear advantage was the formation of easily scalable films injust 20 min with a one-step procedure without any immobilizationrequired. Moreover, it was unprecedented that the CoVO_(x) filmsdeposited on less conductive FTO substrates showed such a high catalyticactivity. This characteristic paves the way for building andunderstanding new catalytic materials, using them in variousapplications besides water splitting reactions, and then moving towardscommercial products.

A rapid one-step aerosol assisted chemical vapor deposition (AACVD)method was employed to synthesize amorphous and highly active CobaltVanadium mixed oxide catalysts (CoVO_(x)) directly over FTO substrates.Morphological and structural characterizations made by FE-SEM, XRD, EDX,and XPS revealed the formation of pure phase amorphous films with agradual variation of topography as a function of deposition time. Themost active of those films, CoVO_(x)-20, was obtained in 20 mindeposition, showing a spongy network of interwoven nanofibers with ahomogeneous distribution of 3-4 nm pores, achieving an overpotential of308 mV at a 10 mA/cm² current density. A much higher current density of175 mA/cm² could be achieved just 380 mV of overpotential with a Tafelslope as low as 62 mV/dec for this whole range while exhibiting longterm stability. Mass activity, EIS data, and the estimation of ECSA allproved this high catalytic performance of CoVO_(x)-20 which isunprecedented for a low cost, up-scalable, and relatively low conductivesubstrate such as the FTO used herein. The findings not only highlightthe benefit of using AACVD in preparing two-dimensional amorphouscatalysts, but also prove the high efficiency of CoVO_(x) materials thusobtained as outlined in a plausible reaction mechanism.

1. (canceled)
 2. The electrochemical water splitting cell of claim 14,wherein the CoVO_(x) layer of the composite thin film electrode isporous with a pore size in a range of 2-10 nm.
 3. The electrochemicalwater splitting cell of claim 14, wherein the composite thin filmelectrode has an electrochemically active surface area in a range of12-22 mF/cm².
 4. The electrochemical water splitting cell of claim 14,wherein the CoVO_(x) layer of the composite thin film electrode consistsessentially of amorphous CoVO_(x).
 5. The electrochemical watersplitting cell of claim 14, wherein the CoVO_(x) layer of the compositethin film electrode has an O:Co molar ratio in a range of 4:1 to 9:1. 6.The electrochemical water splitting cell of claim 21, wherein thesubstrate of the composite thin film electrode is a transparentconducting film selected from the group consisting of fluorine-doped tinoxide, indium tin oxide, aluminum-doped zinc oxide, gallium-doped zincoxide, indium zinc oxide, indium zinc tin oxide, indium aluminum zincoxide, indium gallium zinc oxide, indium gallium tin oxide, and antimonytin oxide. 7-13. (canceled)
 14. An electrochemical water splitting cell,comprising: an electrolyte vessel containing an electrolyte solution, aworking electrode, and a counter electrode, wherein the workingelectrode and the counter electrode are disposed at least partially inthe electrolyte vessel and are at least partially immersed in theelectrolyte solution contained in the electrolyte vessel, wherein theworking electrode is a composite thin film electrode comprising: aCoVO_(x) layer having an average thickness of 500 nm-5 μm in contactwith a substrate, wherein the CoVO_(x) layer comprises amorphousCoVO_(x) having a Co:V molar ratio in a range of 1.0:1.2-1.5:1.0, andwherein the CoVO_(x) layer has an O:Co molar ratio and/or an O:V molarratio in a range of 4:1 to 9:1.
 15. The electrochemical water splittingcell of claim 14, wherein the composite thin film electrode has anoverpotential in a range of 270-335 mV at a current density of 9-11mA/cm².
 16. The electrochemical water splitting cell of claim 14,wherein the composite thin film electrode has a current density of1.0-10.0 mA/cm² when the electrodes are subjected to a bias potential of1.45-1.55 V.
 17. The electrochemical water splitting cell of claim 14,wherein the electrolyte solution comprises water and an inorganic basehaving a concentration of 0.1-1.0 M.
 18. The electrochemical watersplitting cell of claim 14, wherein the composite thin film electrodehas a mass activity in range of 38-50 A/g at a potential of 350 mV.19-20. (canceled)
 21. The electrochemical water splitting cell of claim14, wherein the substrate is a transparent conducting film.
 22. Theelectrochemical water splitting cell of claim 14, wherein the counterelectrode is a platinum mesh electrode.
 23. The electrochemical watersplitting cell of claim 14, wherein the electrolyte solution comprisesat least one inorganic base selected from the group consisting of KOH,LiOH, NaOH, Mg(OH)₂ and Ca(OH)₂.
 24. The electrochemical water splittingcell of claim 14, wherein the composite thin film electrode has a porousnanofiber film structure.